President Obama threatened military action against Syria on Monday if there was evidence that the government of President Bashar Assad was moving its stocks of chemical or biological weapons. It was Obama's most direct warning of American intervention in Syria, where the military is fighting an 18-month-old rebellion.

"We cannot have a situation in which chemical or biological weapons are falling into the hands of the wrong people," Obama said in the White House briefing room. "We have put together a range of contingency plans. We have communicated in no uncertain terms with every player in the region that that's a red line for us."

The president said he was deeply troubled by the prospect that forces loyal to Assad might move or even use the weapons in its increasingly harsh effort to crush the uprising, "That's an issue that doesn't just concern Syria," Obama declared. "It concerns our close allies in the region, including Israel. It concerns us."

Syria is believed to have accumulated huge supplies of mustard gas, sarin nerve agent and cyanide. Assad and other members of his government have said the weapons would not be used except in the case of foreign intervention - a threat that has been interpreted as an attempt to deter any attack by Western nations.

The United States, Obama said, was closely monitoring the situation for any signs that weapons had been moved.

In Syria, Assad's forces stepped up their attacks Monday in and around the southwest city of Daraa, with activists reporting raids, summary executions of suspected opposition figures, and intensified shelling that threatened to reach across the Jordanian border as it did a day earlier, wounding a young girl inside Jordan.

The assault Sunday, criticized by the Jordanian government, seemed to be part of an extended campaign by Syrian forces to regain control of the area, the birthplace of the uprising, which has more recently been the source of several high-profile defectors including the former prime minister, Riad Hijab.